1. Field of the Invention
This invention relates generally to an improved method for the recovery of gold from carbonaceous and pyritic gold-bearing ores. In particular, the ore is preliminarily treated by subjecting it to rapid oxidation achieved through flash-chlorination to render the gold component of the ore more amenable to standard cyanidation treatment.
2. Description of the Prior Art
Sedimentary gold-bearing ores containing indigenous organic carbonaceous material and gold-bearing pyritic minerals are notoriously refractory to standard cyanidation treatment for the recovery of their gold content. Investigation into the cause of this problem has indicated that the carbonaceous materials comprise active carbon and long-chain organic compounds. The active carbon appears to absorb the gold cyanide complex [Au(CN).sub.2 --] from cyanide leaching solutions and the long-chain organic compounds appear to form stable complexes with the gold. In addition, some of these ores also contain gold-bearing sulfides. The sulfides contain gold either as a mechanical inclusion, or as atoms of gold included in the sulfide crystalline lattice.
In order to overcome this sequestering of the gold and to render the gold component of the ore more amenable to standard cyanidation treatment, it has heretofore been proposed that the ore be subjected to a preliminary oxidation treatment to oxidize the carbonaceous, sulfidic mineral materials, and as much of the carbonaceous mineral materials as possible. In experiments conducted by the United States Bureau of Mines, alkaline slurries of carbonaceous gold-bearing ores were subjected to a preliminary oxidation with a wide variety of reagents including ozone, sodium hypochlorite, calcium hypochlorite, permanganates, perchlorates, chlorates and oxygen prior to subjecting the ore to standard cyanidation treatment to extract the gold content therefrom. Of these treatments, the hypochlorites appeared to be the most effective, although it was found that an elevated temperature was required to obtain satisfactory results. At room temperature (about 70.degree. F.) the hypochlorite treatment required several days to make the gold content of the ore available for recovery by cyanidation while temperatures above about 140.degree. F. caused premature decomposition of the hypochlorites. Consequently, it was determined that a temperature within the range of about 122.degree. to 140.degree. F. was required to render the gold component of the ore amenable to standard cyanidation treatment.
U.S. Pat. No. 3,846,124 describes the recoverability, by standard cyanidation, of the gold content of carbon-containing sedimentary gold bearing ores being increased by subjecting the ore to a preliminary oxidation and chlorination treatment in which chlorine gas is introduced into an aqueous slurry of the ground ore having a pH of about 8 to the extent that the slurry will absorb the chlorine, the thus treated slurry being maintained at the chlorination treatment temperature of about 70.degree. to 85.degree. F. for at least 6 hours. The oxidized ore is then subjected to standard cyanidation to extract the gold content therefrom with gold recoveries in the order of 75% or more of the gold content of the ore.
Similarly, U.S. Pat. No. 4,038,362 describes the recoverability, by standard cyanidation, of the gold content of sedimentary gold-bearing ores containing organic carbonaceous material and gold-bearing pyrite, or other gold-bearing sulfides being increased, and the cost of the pretreatment greatly reduced, by subjecting the ore to a two-stage preliminary oxidation and chlorination treatment. In this process, an aqueous slurry of the ore is first heated to about 167.degree. to 212.degree. F. and, thereafter, air or oxygen is introduced into the heated slurry to oxidize and eliminate a substantial portion of the carbonaceous material and oxidizable sulfides in the slurried ore. The slurry is then cooled to about 70.degree. to 85.degree. F., and chlorine gas is introduced into the slurry for a period of 6 to 12 hours and at a pH of 8 or higher to substantially complete the oxidation and chlorination of the carbonaceous and sulfide content of the slurried ore. The oxidized ore is then subjected to conventional cyanidation to recover the gold content therefrom.
Analogously, U.S. Pat. No. 4,259,107 describes a process for treating gold-bearing ores to increase the recovery of gold content therefrom. An aqueous slurry of finely ground ore is first subjected to a preliminary oxidation treatment wherein air or oxygen is bubbled through the slurry while maintaining the slurry at a temperature of from 167.degree. to 212.degree. F. for a period of from 8 to 24 hours. The oxygenated slurry is then cooled to a temperature of from about 70.degree. to 125.degree. F. and acid is added to bring the pH of the slurry down to 6.0 or lower. Chlorine gas or an alkali metal or alkaline earth metal hypochlorite is then added to the slurry until the slurry will no longer absorb or react with these reagents. The thus treated slurry is maintained at a temperature of from 70.degree. to 125.degree. F. for a period of from 6 to 12 hours. The solids content of the slurry is then subjected to standard cyanidation to recover the gold therefrom.
U.S. Pat. No. 4,289,532 describes a process for the recovery of gold from carbonaceous ores. A slurry of carbonaceous gold-bearing ore is oxidized and then subjected to simultaneous cyanidation and granular activated carbon adsorption in two or more solid-liquid extraction stages, where the ore flows countercurrent with the carbon and the gold transfers to the carbon. The chlorination step is carried out at a pH between 5 and 11 or 1 to 6 hours and at a temperature of between 70.degree. and 140.degree. F.
In each of the patents discussed above, it is generally shown that by subjecting a gold-bearing ore to a preliminary chlorination treatment, it is possible to enhance the recovery of gold from such ore during subsequent conventional cyanidation processing.
However, until now, the known methods of chlorination treatment typically involved chlorine gas injections into the slurry or pulp for periods of 1 to 12 hours. Most known techniques involve chlorine gas injections within the upper limits of this range, i.e., between 8 to 12 hours. In addition, the chlorine treatment techniques of the prior art involved the use of large reactors.
The present invention, however, provides a method for the recovery of gold from carbonaceous and pyritic gold-bearing ores and includes an ore pretreatment chlorination step wherein the time required to permit the chlorine gas to become completely adsorbed into the slurry is drastically minimized. In addition, the reactors utilized for the chlorination step are structurally simplified.